In recent years, a speaker that has a parametric effect that utilizes nonlinearity of the air with respect to an ultrasonic wave, and combines a reflecting plate for reflecting a hearable sound wave by the reflecting plate with the speaker has been developed. See e.g., Japanese Patent No. 2,786,531.
In JP 2,786,531 an ultrasonic transducer array is constructed on a concave face of a parabolic substrate having an opening hole in a central portion. The reflecting plate of the hearable sound wave is arranged near the central point of a curvature radius of this substrate. Thus, a secondary wave (hearable sound wave) strong in directivity is reflected on the reflecting plate, and is radiated through a hole opened at the center of the parabolic substrate so that a compact speaker is produced. However, this relates to the ultrasonic transducer having a single sound wave output face. The electrostatic type ultrasonic transducer of the push-pull system having a structure for outputting the sound wave in both face directions of the ultrasonic transducer mainly uses a method in which the sound wave radiated to the rear side is radiated (leaked) as it is, and is attenuated by an absorption material, etc. and is disused. Accordingly, no sound wave emitted to the rear side can be effectively utilized.
FIG. 10 is an explanatory view of a driving concept of the electrostatic type ultrasonic transducer of the push-pull system. In the electrostatic type ultrasonic transducer of the push-pull system, a pair of opposite electrode portions 32a and 32b are arranged to be opposed to a vibrating film 31. A DC bias of the + side is applied to the vibrating film 31 by a DC bias power source, and an alternating current signal is applied between the opposite electrode portions 32a and 32b. 
FIG. 10(a) is a view showing an amplitude state of the vibrating film 31 when the alternating current signal is zero (0) in voltage. The vibrating film 31 is located in a neutral position (the middle of the opposite electrode portions 32a and 32b). FIG. 10(b) is a view showing the amplitude state of the vibrating film 31 when a positive (+) voltage of the alternating current signal is applied to the opposite electrode portion 32a, and a negative (−) voltage of the alternating current signal is applied to the opposite electrode portion 32b. The central portion of the vibrating film 31 is attracted in the direction of the opposite electrode portion 32b by electrostatic force (attractive force) between the vibrating film 31 and the opposite electrode portion 32b, and electrostatic force (repulsive force) between the vibrating film 31 and the opposite electrode portion 32a.
FIG. 10(c) is a view showing the amplitude state of the vibrating film 31 when a negative (−) voltage of the alternating current signal is applied to the opposite electrode portion 32a, and a positive (+) voltage of the alternating current signal is applied to the opposite electrode portion 32b. The central portion of the vibrating film 31 is attracted in the direction of the opposite electrode portion 32a by the electrostatic force (attractive force) between the vibrating film 31 and the opposite electrode portion 32a, and the electrostatic force (repulsive force) between the vibrating film 31 and the opposite electrode portion 32b. Thus, the vibrating film 31 is vibrated in accordance with the alternating current signal and generates a sound wave. The sound wave generated from the vibrating film 31 is radiated in both the face directions of the opposite electrode portions 32a and 32b.
FIG. 11 depicts a working example of the conventional electrostatic type ultrasonic transducer of the push-pull system. When the electrostatic type ultrasonic transducer of the push-pull system (hereinafter also simply called the “ultrasonic transducer”) of the structure for outputting the sound wave in both the face directions is used, the sound wave outputted from both the face sides of the fixing electrode 32 is emitted (leaked) as it is as shown in FIG. 11(a). Otherwise, as shown in FIG. 11(b), the sound wave outputted from the side of one opposite electrode portion 32b is attenuated by an absorption body 70, etc. Accordingly, no ultrasonic transducer is constructed so as to perfectly use the entire sound wave outputted from the ultrasonic transducer.
With respect to the problem discussed above in JP 2,786,531 that the construction is not suitable for perfectly using the entire sound wave outputted from the electrostatic type ultrasonic transducer of the push-pull system, a method for reflecting the sound wave radiated on the rear face of the electrostatic type ultrasonic transducer of the push-pull system and forwards radiating the sound wave by arranging a sound wave reflecting plate on this rear face has been proposed.
FIG. 12 is a view showing an example of the electrostatic type ultrasonic transducer of the push-pull system having the conventional sound wave reflecting plate, and in which the sound wave reflecting plate 42 is arranged on the rear face of the ultrasonic transducer 30. However, in this construction, when the outside diameter of the ultrasonic transducer is set to R2, it is necessary to set the outside diameter (R1) of the sound wave reflecting plate required here to twice or more the outside diameter R2 of the ultrasonic transducer. It is also necessary to set the area of the sound wave reflecting plate to four times or more the area of the ultrasonic transducer. Here, when R1=2·R2 is set, the area of the sound wave radiating face of the ultrasonic transducer is “(¼)·π·(R1)2”, and the area of the sound wave reflecting plate is “π·(R2)2”.
Thus, the outside diameter of the ultrasonic speaker is determined by the outside diameter of the sound wave reflecting plate, and a region for generating an ultrasonic wave with respect to its size is ¼ in area and is therefore very narrow so that area efficiency is bad. Further, this large arranging space became a factor of difficulty of assembly into a video image or a television device, etc.